1. Field of the Invention
This invention relates to a motor control system which is adapted to control the operations of a plurality of servomotors.
2. Related Art
Numerical control systems or positioning systems for controlling industrial robots, machine tools, automated devices, and so forth, employ AC servomotors to position the rotary shafts. The servo-motor is controlled by a motor control device using a servo control system.
FIG. 12 shows an external arrangement of a conventional motor control system for controlling a plurality of servomotors. The motor control system shown in FIG. 12 is for six servomotors. Control units U1 through U6 are provided for the six servomotors, respectively. Those control units U1 through U6 are juxtaposed together with a higher control unit Uh which is adapted to transmit control instructions from an external controller. The control units U1 through U6 and the higher control unit Uh are accommodated in their own casings, respectively, and have a number of electrical wiring connectors and so forth on the front walls.
Those connectors are roughly classified into three kinds of connectors--power wiring connectors, signal line connectors, and motor driving connectors. Each of the control units (for instance the control unit U6) has a serial communication connector 204, a control signal connector 205, an encoder connector 206, a motor drive line connector 207, and a power connector 211. Hence, it is necessary to connect a common power source 200 through wires to the power connectors 211 of all the control units U1 through U6, and to connect servomotors M1 through M6 through wires to motor drive line connectors 207, and to connect encoders E1 through E6 through wires to the encoder connectors 206. In addition, in order to transmit control signals, it is necessary to extend jumper wires from the higher control unit Uh to the control signal connectors 205 of the control units U1 through U6. Furthermore, in each of the control units U1 through U6, wiring must be made for other controls.
FIG. 13 shows a typical example of the control units, namely, the control unit U1. In FIG. 13, a rotational position target value Pref of the servomotor M1 is applied through the control signal connector 205 to a position control section 260. In the position control section 260, the rotational position target value Pref is compared with a position detection value which is obtained from an encoder signal e1 fed back from the encoder E1, and the difference between them is applied, as a speed instruction s, to a speed control section 261. In the speed control section 261, the speed instruction s is compared with the speed detection value obtained from the encoder signal e1, and the difference between them is applied, as e current instruction i, to a current control section 262. The current control section 262 forms a three-phase instruction v according to a magnetic pole position signal of the servo motor M1 obtained from the encoder signal e1, the current instruction i, and a current detection value Idet back from a current detector 258 and an A/D converter 253, and applies it to a PWM signal generating section 263. The PWM signal generating section 263 forms a PWM signal according to the three-phase instruction, and applies it through a dead time forming section 251 to an inverter 252. The latter 252 generates a motor drive voltage according to the PWM signal, to drive the servomotor M1.
The above-described current detector 258 forms a feedback loop to monitor the drive current of the inverter 252 for feedback control, and its output signal is applied to the AD converter 253, where it is subjected to A/D (analog-to-digital) conversion.
A conventional servo control system of this type has been disclosed, for instance, by Unexamined Japanese Patent Publication No. 90011/1992.
As is apparent from the above description, the conventional motor control system for controlling a plurality of servomotors needs a number of wiring cables and accordingly a number of components relevant thereto. Therefore, the system suffers from problems that it is high in component cost, and complicated in wiring, and accordingly high in assembling cost. In addition, the intricate wiring may result in the occurrence of wiring errors, which may lower the work efficiency in the assembling of the system.
If any one of the control units U1 through U6 becomes out of order, the repair is troublesome: First, from each of the control units, the connectors 205, 206, 207 and 211 are removed, and then the control unit is replaced with a new one. Thereafter, the wires must be connected to the connectors 205, 206, 207 and 211 again. This work is considerably laborious.